1. Field of the Invention
This invention relates to a process for extracting hydrocarbons from the earth. More particularly, this invention relates to a method for recovering viscous hydrocarbons such as bitumen from a subterranean reservoir by continuously injecting a heated fluid to lower the viscosity of the viscous hydrocarbons concurrent with production of mobilized hydrocarbons.
2. Description of the Prior Art
In many areas of the world, there are large deposits of viscous petroleum, such as the Athabasca and Cold Lake regions in Alberta, Canada, the Jobo region in Venezuela and the Edna and Sisquoc regions in California. These deposits are often referred to as "tar sand" or "heavy oil" deposits due to the high viscosity of the hydrocarbons which they contain. These tar sands may extend for many miles and occur in varying thicknesses of up to more than 300 feet. Although tar sand deposits may lie at or near the earth's surface, generally they are located under a substantial overburden which may be as great as several thousand feet thick. Tar sands located at these depths constitute some of the world's largest presently known petroleum deposits. The tar sands contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount which ranges from about 5 to about 20 percent by weight. Bitumen is usually immobile at typical reservoir temperatures. For example, in the Cold Lake region of Alberta, at a typical reservoir temperatures of about 55.degree. F., bitumen is immobile with a viscosity exceeding several thousand poises. However, at higher temperatures, such as temperatures exceeding 200.degree. F., the bitumen generally becomes mobile with a viscosity of less than 345 centipoises.
Since most tar sand deposits are too deep to be mined economically, a serious need exists for an in situ recovery process wherein the bitumen is separated from the sand in the formation and produced through a well drilled into the deposit. Two basic technical requirements must be met by any in situ recovery process: (1) the viscosity of the bitumen must be sufficiently reduced so that the bitumen will flow to a production well; and (2) a sufficient driving force must be applied to the mobilized bitumen to induce production. Among the various methods for in situ recovery of bitumen from tar sands, processes which involve the injection of steam are generally regarded as most economical and efficient. Steam can be utilized to heat and fluidize the immobile bitumen and, in some cases, to drive the mobilized bitumen towards production means. Indeed, a majority of the processes currently employed utilize the injection of steam in one form or another.
Several steam injection processes have been suggested to heat the bitumen. One general method for recovering viscous hydrocarbons is by using "steam stimulation" techniques, the most common being the "huff and puff" process. In the process, steam is injected into a formation by means of a well and the well is shut-in to permit the steam to heat the bitumen, thereby reducing its viscosity. Subsequently, all formation fluids, including mobilized bitumen, water and steam, are produced from the well using accumulated reservoir pressure as the driving force for production. Initially, sufficient pressure may be available in the vicinity of the wellbore to lift fluids to the surface; as the pressure falls, artificial lifting methods are normally employed. Production is terminated when no longer economical and steam is injected again. This cycle may take place many times until oil production is no longer economical.
In the huff-and-puff method the highest pressures and temperatures exist in the vicinity of the well immediately following the injection phase. Normally this pressure and temperature will correspond to the properties of the steam which was employed. Before oil can be moved from the remote parts of the reservoir to the well, the pressure in the near well region must fall so it is lower than the distant reservoir pressure. During this initial depressuring phase, the near wellbore reservoir material cools down as water flashes into steam. The first production from the well thus tends to be steam and this tends to be followed by hot water. Eventually the pressure is low enough and oil can move to the wellbore. In the initial production phase, much of the heat which was put into the reservoir with the steam is simply removed again as steam and hot water. A major inefficiency of the huff-and-puff process is that this heat must be supplied during each cycle and as the available oil becomes more remote from the well, this cyclic wasted heat quantity increases.
The principal drawbacks of the "huff and puff" process, therefore, are: (1) production is not continuous, (2) the majority of the bitumen in the reservoir is never heated, thereby limiting recovery, and (3) the production cycle inherently removes most of the heating medium from the formation, and consequently much of the heating value of the injected steam is wasted.
A second general method for recovering viscous hydrocarbons is by using "thermal drive" processes. Typically, thermal drive processes employ an injection well and a production well, spaced apart from each other by some distance and extending into the heavy oil formation. In operation, a heated fluid (such as steam or hot water) is injected through the injection well. Typically entering the formation, the heated fluid convectively mixes with heavy oil and lowers the viscosity of the heavy oil, which is mobilized and driven by the heated fluid towards the production well. One advantage in using a thermal drive process is that higher recoveries may be obtained. For example, it has been the general experience in California that higher thermal efficiencies are achieved with steam stimulation, but that only relatively low recoveries are obtained overall. With steam floods, the recovery is higher, although more heat is used per barrel of produced oil.
Unfortunately, the general experience of industry has been that conventional thermal drive processes are not commercially effective in recovering bitumen from tar sands. One basic problem is that there is a restricted fluid mobility due to the high viscosity hydrocarbons cooling as they move through the formation; these cooled hydrocarbons build up away from the injection well to create impermeable barriers to flow. Another serious problems is that often the driving force of the flowing heated fluid is lost upon breakthrough at the production well. Fluid breakthrough causes a loss of driving pressure and a marked drop in oil production. In addition, much of the heating value of the heated fluid is lost upon breakthrough.
Various steam stimulation and thermal drive methods have been proposed in the prior art. For example, U.S. Pat. No. 2,881,838 to R. A. Morse et al discloses a method for recovering viscous hydrocarbons wherein a single well is drilled through the producing formation; steam is then injected via the well into the upper portion of the formation to mobilize the viscous hydrocarbons which flow by gravity drainage to the bottom of the well; these mobilized hydrocarbons are then pumped to the surface. Steam is injected at rates calculated to continuously expand a heating zone in the formation as the mobilized heavy oil flows to the bottom of the well and is produced, but at the same time at rates which avoid substantial steam bypassing. A major disadvantage with Morse's process is that it contemplates only a radial process slowly growing from a vertical well. In such an operation, the heated surface during the initial stages is very small and only extremely low production rates are achieved.
In Morse, steam is introduced down the annulus of a well and liquids are produced up a central tubing. For this to be operable, it is necessary that at each point in the vertical well the steam be at a lower pressure than the pressure of the liquids in the inner tubing. If this is not the case, then heat will be transferred from the annulus through the tubing, condensing steam in the annulus, and boiling water in the tubing. This would be very wasteful. The Morse patent also suggests that a pump at the base of the well be able to overcome the hydrostatic head of liquid to the surface. In practice, it will also have to develop an additional pressure at the surface at least equal to the pressure of the injected steam, which may be uneconomical. The Morse patent also describes operation without a pump. If this were tried with the apparatus shown, then the pressure in the tubing would have to be less than the pressure in the annulus and excessive condensation of steam and flashing of water in the tubing would occur.
The Morse patent also does not recognize a problem which can arise from the evolution of non-condensable gas (natural gas) from the oil as it is heated. This non-condensable gas will mix with the steam and tend to accumulate near the interface. This will hinder the movement of the steam from the chamber to the interface where it is desired to condense it.
Yet another difficulty with the Morse process is that it recommends the use of a perforated and cemented casing for injection. A significant pressure drop would be required to cause injection of steam at practical rates from such a casing. This pressure difference would also be exerted at the bottom of the casing and would tend to prevent oil draining to the central production tubing.
U.S. Pat. No. 3,960,214 to Striegler et al discloses another approach which involves drilling a horizontal injection well and positioning several vertical production wells above and along the length of the injection well. A heated fluid is circulated through the horizontal well to contact the formation, mobilizing the bitumen which is then recovered through the vertical production wells. A problem sought to be addressed by this patent is that of providing a permeable, competent communication path between injection and production wells, thereby avoid the problems of cooled bitumen banking up to create impermeable barriers to flow. However, the mechanism of recovery is not clear and clearly does not depend on gravity drainage of heated oil.
Another example of a thermal drive method for continuously producing viscous mobilized viscous hydrocarbons is Canadian Pat. No. 1,028,943 to J. C. Allen. This patent proposes that prior to injecting steam into a formation, a non-condensable and non-oxidizing gas be injected to establish an initial gas saturation. Following this, a mixture of steam and non-condensable gas are injected. By utilizing this method, it is said that pressure communication between an injection well and a production well can be maintained and also premature pressure decline is avoided. Flow of oil from one well to the other is caused by lateral pressure differences; a gravity drainage process is clearly not involved.
Major problems still exist with each of these processes, in particular, and with thermal drive processes in general. One problem stems from the fact that the injected steam condenses and mixes with the mobilized bitumen as these fluids move through the formation. Any significant mixing of the mobilized heavy oil and condensed water results in a greatly reduced oil relative permeability. A second problem is that low steam injection pressures are often required to avoid the formation of fractures within a reservoir. However, at such pressures, it may not be possible to inject steam having enough heating value to economically heat the formation and mobilize the bitumen. A third problem is that as steam injection continues and the reservoir is heated, non-condensable gases contained in the formation will fractionate and accumulate in the reservoir. If this occurs to a significant extent, oil production can decline and stop due to a pressure buildup which counteracts oil flow.
Therefore, while the above methods are of interest, the technology has not generally been economically attractive for commercial development of tar sands. Substantial problems exist with each process of the prior art. Therefore, there is a continuing need for an improved thermal process for the effective recovery of viscous hydrocarbons from subterranean formations such as tar sand deposits.